Accompanied with the development of high-capacity semiconductor memories and the development of high-speed or highly integrated CPU processors, finer photolithography has become indispensable.
In general, the lower limit of fine processing in a photolithographic apparatus is about one third of the wavelength of a light source used.
Therefore, the shortening of the wavelength of light used for a photolithographic apparatus has been attempted, and it is now possible to perform fine processing of about 50 nm.
Although the finer photolithography has been developed as described above, with the shortening of the wavelength of light used, there have been posed many problems to be solved, such as increase in the size of an exposure apparatus, development of a lens for such a shorter wavelength, costs for the apparatus, and costs for a corresponding resist.
On the other hand, in order to carry out fine processing with a resolution less than the wavelength of used light, a method using near field light has been proposed.
Since the near field photolithography is not restricted by a diffraction limit of light, a spatial resolution which is not more than one third of the wavelength of a light source can be obtained.
In addition, by using a mercury lamp or semiconductor laser as a light source, the light source itself can be reduced in size, thereby enabling reduction in size of the structure of an exposure apparatus as well as reduction in costs.
As an example of such methods using near field light, there is known a method of performing scanning with a probe prepared by sharpening a tip of an optical fiber by wet etching (Japanese Patent Application Laid-Open No. H07-106229).
However, in this method, fine processing is performed in a manner similar to drawing with one stroke by use of one or more processing probes, so that the method has a problem that the throughput needs to be improved.
Therefore, there has been proposed a near field exposure method in which a photomask with a light shielding layer having formed therein an opening which is less in size than the wavelength of a light source is brought into close contact with a resist and one-shot exposure is performed, thereby improving the throughput (Appl. Phys. Lett., 75, 3566 (1999) and U.S. Pat. No. 6,171,730).
In order to form a fine resist pattern using near field light by employment of such a close contact exposure system, it is necessary to use a near field exposure mask with a light shielding layer having an opening pattern which is less in size than the wavelength of exposure light and to bring the near field exposure mask and a resist into close contact with each other.
This is because the intensity distribution of near field light is attenuated rapidly with increasing distance from the fine opening.
By using silicon for a light shielding layer of a near field exposure mask, a large extinction coefficient is obtained and the fine processing by dry etching can be easily performed, thereby enabling formation of a fine light shielding layer pattern.
Hitherto, there has been known a technology of forming a fine resist pattern by transferring a mask pattern by exposure using a near field exposure mask having such a light shielding layer (Extended Abstracts of the 53rd Spring Meeting of Japan Society of Applied Physics, 25a-ZB-1 (2006))
In addition, Japanese Patent Application Laid-Open No. 2001-166453 discloses a technology such that in a projection exposure mask, a reflective layer is provided between a metal light shielding layer and a mask matrix to thereby suppress a thermal strain of the projection exposure mask itself.
Meanwhile, in the near field exposure in which exposure is performed with a mask having a light shielding layer being in close contact with a resist layer, there is posed a problem that heat of the mask resulting from the light shielding layer is transferred to the resist layer, whereby the pattern size varies.
Next, this problem will be described in more detail.
In the near field exposure, a light shielding layer of a mask reflects or absorbs irradiation light, thereby forming a dark portion immediately therebelow.
Here, description will be made by taking, as an example, a case where an i-line (wavelength: 365 nm) of a mercury lamp is used as exposure light, silicon nitride (complex refractive index with respect to i-line: 2.09+0i) is used as a mask matrix, and amorphous silicon (complex refractive index with respect to i-line: 3.90+2.66i) is used as a light shielding layer.
In this case, the reflectance of the exposure light at an interface between the mask matrix and the light shielding layer calculated using the Fresnel's formulas is 21%.
The exposure light which has not been reflected is absorbed by the light shielding layer and is converted into heat.
It is needless to say that the transmittance of the light shielding layer of the photomask is about 0%.
On the other hand, a resist exhibits development contrast mainly through a photoreaction, but it is generally known that the reaction is promoted also by heat.
Particularly, in those resists which cause a reaction using, as a catalyst, an acid generated from a photoacid generator, such as a chemically amplified type resist and a photoinitiated cationic polymerization type resist, the promotion of the reaction due to heat is remarkable.
For that reason, in the near field exposure in which exposure is performed with a mask having a light shielding layer being in close contact with a resist layer, the heat of the mask which has been generated in the light shielding layer is transferred to the resist layer, which varies the reaction rate of the resist depending on the temperature of the mask, whereby a variation is generated in the pattern size for each shot.
At that time, when the reaction rate of the resist according to the temperature of the mask is high, for example, in a case of a line pattern, the line width is small when a positive resist is used, and the line width is large when a negative resist is used.
Such a problem has not been posed in projection exposure and proximity exposure in which a mask and a resist are not brought into contact with each other, and is peculiar to the near field exposure in which exposure is performed with a mask having a light shielding layer being in close contact with a resist layer.